Paper is manufactured on a papermaking machine as a continuously formed web of paper. A papermaking machine has a wet end where the web is formed by depositing paper forming fibers from a dilute stock of fibers suspended in water. The fibers are deposited onto a forming section or screen in what is known as the forming process. Water is removed from the forming wires by drainage through these screens which is often aided by suction boxes located beneath the forming wire.
As the web leaves the forming wires it passes beneath a suction pick-up roll. The web leaves this roll and the forming section containing approximately 20 percent fiber by weight and enters the press section of the papermaking machine. In the press section of the papermaking machine the paper web fiber content is increased to a level of about 40-50 percent fiber by weight by pressing the web to remove water. From the press section the web enters the drying section where a series of steam heated dryer rolls dry the paper to about 95 percent fiber by weight. After the paper has passed through the drying section the web is subjected to one or more calenders to smooth and improve the surface finish of the web. The web is then wound into a reel for removal from the papermaking machine.
The press section of a papermaking machine is typically composed of two or three nips, and may employ a third nip of the Extended Nip press type. An Extended Nip press employs a concave shoe which engages a blanket and the web against an opposed roll. A lubricant film is disposed between the blanket and the shoe which supports the blanket in its passage through the Extended Nip press. The nip of an Extended Nip press is typically about ten inches wide and the backing roll is in some configurations heated to between 300 and 700 degrees Fahrenheit.
A particular papermaking machine will often need to be designed to produce a particular or narrow range of paper types from a particular stock. One type of stock which can be particularly difficult to work with is stock make from 100 percent tropical hardwoods. Although temperate forests are dominated by just a few species of trees, tropical forests contain a large number of different species which invariably must be harvested together. Even where managed forests are harvested, a large number of tropical hardwoods species will typically be grown together to maintain a diverse ecosystem and minimize the susceptibility of the forest to disease and insect damage. Thus the properties of tropical hardwoods pulp can be difficult to characterize. Tropical hardwood pulp contains a fair amount of resinous particles which can cause problems in the drying section of a papermaking machine. A web formed from tropical hardwood fibers can stick excessively to the press rolls. The tropical hardwood pulp fibers are weak and this can increase the number of web breaks experienced in a typical machine day.
What is needed is a pressing section for a papermaking machine designed to improve the performance of a papermaking machine working with selective paper forming stocks including 100 percent tropical hardwood fibers.